The Habits and Characteristics of Nocturnal Animals

Abstract

A discussion of the conditions of life at night, possible advantages of the nocturnal habit, and the usefulness at night of the several senses. The degree of activity of nocturnal animals is determined chiefly by light intensity, but also by other factors, e.g., temp., humidity and rate of evaporation. Nocturnal rhythms, once established, may persist under constant environmental conditions. Possible advantages of the habit include the evading of daytime enemies, easier obtaining of preferred food, avoidance of excessive evaporation from the body, and easier communication between individuals. Adaptations to nocturnal life are chiefly the unusual refinements of one or more of the senses, and are differences of degree rather than of kind. Excepting luminescence and some adaptations of vision, there seems to be no modification that might not be fully as useful in light as in darkness. Light production at night probably has some part in congregating and sex attraction and may serve as a warning signal. Adap-tations for nocturnal vision among invertebrates include division of the eye into 2 parts, pigment migration, and the presence of a reflecting tapetum. Keenness of vision at night among vertebrates depends on the possession of a rod-rich retina, pigment migration and rod shortening, and often is aided by a reflecting tapetum. These processes may be accompanied or largely replaced by accumulation of visual purple and by iris dilation. Nocturnal animals frequently have relatively large eyes. Frequently, the iris has a vertical aperture. Typical nocturnal vertebrates lack the yellow or red intra-ocular filters characteristic of diurnal forms.